In a principal aspect the present invention relates to a methods of reinforcing pipes and in particular repairing embedded pipes.
These pipes belong for example to a water delivery network. In their most common form these pipes are constituted by an end to end assembly of segments of pipe made from pre-stressed concrete and having a relatively large diameter of typically 0.5 m to 6 m environ. Water flows through said pipes under a pressure of up to around twenty bars. These segments of concrete pipe can incorporate an interior metal cylinder (liner). The latter is not designed, however, to carry the stress in terms of pressure due to the fluid flowing through. This stress is carried by passive armatures (pre-fabricated cylindrical armature cage or by the pre-stressing of the concrete. The latter is realized by means of threads wound in a spiral outside of the concrete core of the pipe segment. During manufacture of this segment its concrete core is rotated about its axis to receive the thread which is checked in order to be brought under pressure. This thread is then protected against corrosion by projecting a supplementary layer of concrete or mortar over twenty to thirty millimeters.
FIG. 1 shows the structure of a pre-stressed concrete pipe as used in many water delivery networks. The wall of the pipe comprises:                a concrete core 11 incorporating a watertight liner 12 constituted by a ductile cylinder made from thin steel (so-called embedded-cylinder pipe). The concrete of the core 11 is distributed between an internal layer 13 of a few centimeters in thickness which is situated on the internal side of the pipe and a thicker external layer 14 which in most cases is not reinforced;        pre-stressing threads wound in one or two layers around the concrete core;        protective anti-corrosion mortar 16 surrounding the threads 15 in order to passivate them;        an optional paint or resin 17 improving the anti-corrosion protection.        
Pipes of this type are most frequently embedded. They are exposed to a risk of corrosion according to the aggressiveness of the environment. The most common form of degradation occurs as follows:                migration of aggressive ions across the protective mortar 16;        corrosion of the pre-stressing threads 15;        breaking of the pre-stressing threads and local delamination;        general delamination of the external mortar 16;        depassivation of the threads and acceleration of corrosion.        
The process can then accelerate and lead to the breaking of the pipe. Methods of magnetic or acoustic detection allow breaks in thread to be located and the state of the pipe to be assessed. Depending upon the assessed state a decision to repair can be taken.
Repair is generally carried out from the exterior. New armatures, passive or pre-stressed, are placed around the pipe in order to collar it. See for example international patent application published as WO 03/014614.
Repairs can also be carried out from the inside by incorporating within the pipe a resistant core for the purpose of re-establishing the resistance lost through the breaking of the pre-stressing threads. Repairs carried out from the inside are generally more expensive. Such repairs are carried out when the excavation works are impossible or difficult.
Repairs from the inside generally consist in placing a metal liner within the pipe. The interstice between the new resistant liner and the pipe is filled with grout. One difficulty is that a local error in realization can lead to infiltrations of water in the injected interstice. This water penetration can cause the concrete core of the pipe to be brought under pressure and can make the new resistant liner inefficient. This will result in a risk of a break in the pipe outside of the new liner.
When repairing from the inside the reinforcements added cannot be pre-stressed. When the pipe is brought under pressure these reinforcements undergo a certain elastic deformation (increase in their diameter) which causes cracking of the concrete of the core of the pipe. The concrete breaks in traction for a very small deformation, no material having the capacity to carry the significant stress for such extension. The cracking of the concrete causes two disadvantages. It worsens the flexural strength of the pipe and hence its capacity to resist dissymmetrical thrusts of the earth. It also allows corrosion to progress from the outside environment to the inside of the pipe. It also allows corrosion to progress from the exterior to the interior of the pipe. This risks causing early corrosion of the new reinforcement if it is metal. This problem often leads to the solutions for repairs from the inside with a basis of metal liners to be dismissed.
It is an object of the present invention to propose a perfected method of carrying out repairs from the inside.